What is the output resistance of a common - emitter amplifier?

Hey there! As a transistor supplier, I often get asked all sorts of questions about transistors and their applications. One question that comes up quite a bit is about the output resistance of a common - emitter amplifier. So, let's dive right in and break it down.

First off, let's quickly go over what a common - emitter amplifier is. A common - emitter amplifier is one of the most widely used transistor amplifier configurations. In this setup, the emitter terminal of the transistor is common to both the input and the output circuits. It's known for its high voltage gain, moderate input impedance, and relatively high output impedance.

Now, let's talk about output resistance. Output resistance is a crucial parameter when it comes to amplifiers. It basically represents how the amplifier behaves when it's connected to a load. You can think of it as an equivalent resistance that the amplifier presents to the load connected to its output.

For a common - emitter amplifier, the output resistance is mainly determined by two factors: the Early effect and the load resistance connected to the collector.

The Early effect is a phenomenon in bipolar junction transistors (BJTs). When you increase the collector - emitter voltage (VCE) of a BJT, the width of the base - collector depletion region increases. This, in turn, reduces the effective base width. As the base width decreases, the number of carriers recombining in the base region decreases, which leads to an increase in the collector current. This relationship between the collector current and the collector - emitter voltage is described by the Early voltage (VA).

Mathematically, the output resistance (ro) of a BJT due to the Early effect can be expressed as:

ro = VA / IC

where VA is the Early voltage and IC is the DC collector current. The Early voltage is a characteristic parameter of the transistor and can be found in the transistor's datasheet.

Now, when you have a load resistance (RL) connected to the collector of the common - emitter amplifier, the total output resistance (Rout) of the amplifier is given by the parallel combination of ro and RL. That is:

1/Rout = 1/ro + 1/RL

or

Rout = (ro * RL) / (ro + RL)

Let's take a look at an example to better understand this. Suppose we have a transistor with an Early voltage VA = 100 V and a DC collector current IC = 1 mA. First, we calculate the output resistance due to the Early effect:

ro = VA / IC = 100 V / 1 mA = 100 kΩ

Now, let's say we connect a load resistance RL = 10 kΩ to the collector. We can calculate the total output resistance of the amplifier as follows:

Rout = (ro * RL) / (ro + RL) = (100 kΩ * 10 kΩ) / (100 kΩ+ 10 kΩ) ≈ 9.09 kΩ

The output resistance of a common - emitter amplifier has several implications. A high output resistance means that the amplifier can deliver a large voltage gain. However, it also means that the amplifier is not very good at driving low - impedance loads. When you connect a low - impedance load to an amplifier with high output resistance, a significant amount of the output voltage will be dropped across the output resistance, resulting in a reduced voltage across the load.

On the other hand, a low output resistance allows the amplifier to drive low - impedance loads more effectively. The voltage across the load will be closer to the output voltage of the amplifier, resulting in better power transfer to the load.

As a Transistor supplier, I understand the importance of choosing the right transistor for your amplifier design. Different transistors have different Early voltages and other characteristics that can affect the output resistance of your common - emitter amplifier. That's why we offer a wide range of transistors with various specifications to meet your specific needs.

When you're designing a common - emitter amplifier, it's essential to consider the output resistance requirements based on your application. If you're building an amplifier for a high - impedance load, like a pre - amplifier for a high - impedance microphone, a transistor with a relatively high output resistance might be a good choice. On the other hand, if you need to drive a low - impedance load, such as a speaker, you'll want to select a transistor that can provide a lower output resistance.

We also provide detailed datasheets for all our transistors, which include information about the Early voltage and other parameters that can help you calculate the output resistance of your amplifier. Our technical support team is always ready to assist you with any questions you might have regarding transistor selection or amplifier design.

If you're in the process of designing a common - emitter amplifier or any other transistor - based circuit, I encourage you to reach out to us. We can help you choose the right transistor that will give you the desired output resistance and performance. Whether you're a hobbyist working on a small project or an engineer designing a large - scale electronic system, we have the products and expertise to support you.

So, don't hesitate to get in touch with us to start the procurement process and discuss your specific requirements. We're looking forward to working with you and helping you achieve the best results in your amplifier designs.

References

• Sedra, A. S., & Smith, K. C. (2015). Microelectronic Circuits. Oxford University Press.
• Boylestad, R. L., & Nashelsky, L. (2013). Electronic Devices and Circuit Theory. Pearson.